A variety of instruments which produce depth resolved information and imaging of the eye, tissue or industrial objects are known. They involve CM and OCT principles. A general problem with all these configurations is their large volume and high cost. This restricts their use to a few medical practices and research centres.
In the description which follows, reference is made primarily to the human eye and skin, however the invention is also applicable to measurements and imaging of any other objects which are sufficiently transparent for visible or infrared light, as well as for profilometry of any object which reflects visible or infrared light. In terms of imaging moving objects, reference is made primarily to two examples, live embryos or the retina of an eye. This has to be understood as merely a way to help the description and not as a restriction of the application of the present invention. As such, where the term “embryo” or “eye” or “retina” is used, a more general transparent and scattering object or organ may be sought instead, the invention could equally be applied to skin, heart, vessels, dental tissue, dental prostheses, paintings, powders and other scattering semi-transparent objects, moving or non-moving. The dual imaging aspect of the invention is especially useful for moving objects.
All known imaging systems for the eye, such as fundus cameras, slit cameras, SLO and OCT systems as well as imaging systems for skin, other tissue, profilometry use an optical source, scanners, some optics and a reader, in the form of photodetector devices or 1D or 2D photodetector arrays.
However, all these systems are inaccessible to most of the small medical practices and small businesses due to their high cost. Some use sophisticated optical sources, such as femtosecond lasers pulsed or CW lasers, specialized sources such as superluminiscent diodes (SLD). These optical imaging systems also use specialized 1D and 2D photodetector arrays, or many pixels, high dynamic CCD or CMOS cameras, of high cost.
The implementation of such systems on specialized chin rests for imaging the eye or microscopy on highly specialised frames lead to a further increase in their price.
There are instances when home, cross section or en-face images would be needed of translucent objects, such as nails, sheets of paper, objects of ceramics or porcelain, etc. A 1D profile of reflectivity in the anterior chamber of the eye could be used to evaluate the sugar content eliminating the need of the current invasive measurement methods using a drop of blood. The bulky format of available OCT systems and their cost prevent OCT technology from replacing invasive methods, or penetrating the consumer market, to the level and extent of electrical appliances. High skills are required to operate such OCT systems, at the level of a well educated Physicist, engineer or medical practitioner. They are sophisticated and complex and cannot be handled in the way a PC or an electrical appliance is used by any consumer.
Due to the reasons mentioned above, it is hard to imagine that such sophisticated imaging systems would have a wide spread in the countryside and small towns where authorities struggle in ensuring provision of even basic medical devices. Also, small companies cannot afford to purchase such systems for profilometry or topography, distance measurement, or correlation measurements due to their high cost.
In the battle field, ophthalmologists need OCT systems to evaluate eye damage. Art conservationist need OCT systems in harsh environments, in either very hot conditions or very cold. Underwater inspection of relics by conservationists or salvage teams is another example where portable and compact high resolution instruments are needed. The bulky systems known today cannot be easily made transportable or adapted for harsh conditions.
Therefore, a need exists for more compact high depth resolution measurement and imaging systems, portable and of much lower cost to allow the spread of confocal microscopy technology and of OCT technology to satisfy the needs of ordinary people without resorting to specialized equipment or advice, or to satisfy the needs of specialists working in harsh weather conditions or difficult environment and to be used by small practices in diagnostic as well as by small companies in industry.
There are also known imaging and measurement instruments using monochrome high performance cameras. The imaging of moving organs or objects or fast evolving phenomena is often difficult due to the time required to collect repetitive data for different values of polarisation, wavelength or angular incidence for polarisation, spectroscopic and speckle reduction imaging respectively.
Therefore a need exists to speed up the acquisition by conveniently and advantageously employing the novel features available in modern digital cameras. A form of spectral domain OCT, called channelled spectrum (CS) of Fourier domain OCT is based on reading the channelled spectrum at the output of an interferometer using a spectrometer, as described in “Displacement Sensor Using Channeled Spectrum Dispersed on a Linear CCD Array”, by S. Taplin, A. Gh. Podoleanu, D. J. Webb, D. A. Jackson, published in Electron. Lett. 29, No. 10, (1993), pp. 896-897 and in “Channelled Spectrum Liquid Refractometer”, by A. Gh. Podoleanu S. Taplin, D. J. Webb, D. A. Jackson, published in Rev. Sci. Instr., vol. 64, No. 10, pp. 3028-9, (1993). By adding a transversal scanning head to the configuration described in these two papers, OCT functionality is achieved. However, such methods produce B-scan OCT images only. It will be desirable to have an en-face image to guide the B-scan acquisition of moving embryos, organs or any other moving samples. It will also be useful to see the eye fundus when cross-sectioning the retina. Fourier domain optical coherence tomography systems are based on an interferometer whose spectrum is read by a linear CCD array. Increase in the speed and dynamic range of digital linear cameras allowed progress in this field. Cameras with 2048 pixels which could be read at more than 100 kHz line rate are now available. SLR cameras with more than 1000×1000 pixels and with a 10 microsecond acquisition times are also available, which shows that the performance of commercially available cameras improved to the level of scientific more expensive cameras.
OCT has mainly evolved in the direction of producing cross-sectional images, most commonly perpendicular to the plane of images delivered by a microscope or by a SLO. The depth resolution in SLO is 30-100 μm coarser than that in OCT while the transversal resolution in OCT is affected by random interference effects from different scattering centers (speckle), inexistent in SLO images. Therefore, there is scope in combining SLO with OCT. Different avenues have been evaluated, to provide an SLO using CS-OCT systems. The main motivation for OCT/SLO combination is to provide orientation to the OCT channel. Crucial for the operation is pixel to pixel correspondence between the two channels, OCT and SLO, which can only be ensured if both channels share the same transverse scanner to scan the beam across the eye.
An equivalent SLO image can be generated from several OCT B-scans. Then, by software means, an SLO image can be inferred without using a beamsplitter or a separate confocal receiver. After a 3D data set acquisition, a confocal microscopy image of the embryo (or an SLO-like image of the retina is generated) and then the B-scan OCT images can be revisited through the 3D data set with simultaneous display of the synthesized CM (or SLO) image. SLO-like image cane be inferred from B-scans using CS-OCT systems, as reported in Hong, Y., Makita, S., Yamanari, M., Miura, M., Kim, S., Yatagai, T., Yasuno, Y 2007, “Three-dimensional visualization of choroidal vessels by using standard and ultra-high resolution scattering optical coherence angiography”, published in Opt. Express 15, 7538-7550 or by Jiao, S. L., Wu, C. Y., Knighton, R. W., Gregori, G., Puliafito, C. A., 2006, “Registration of high-density cross sectional images to the fundus image in spectral-domain ophthalmic optical coherence tomography”, Published in Optics Express 14, 3368-3376.
The main advantage of the spectral OCT method relies on its high speed which allows collection of a large data set of pixels. With a high density of 65536 A-scans, obtained at 29 kHz, 2.25 s are required for the whole volume. The transversal resolution along the synthesis axis of the SLO image is given by the spatial sampling, i.e. by the lateral interval from a B-scan to the next B-scan along a rectangular direction to that contained in the B-scan image. Such SLO-like C-scan images exhibit the normal transversal resolution (15-20 □m) along the B-scan lateral coordinate (X) and the coarse sampling interval, along the lateral rectangular direction (Y). For instance, let us say that the image is from an area of 4 mm×4 mm on the retina of 512×128 pixels. This means that the Y-pixel size is 4 mm/128=31 □m. This size could be reduced by increasing the acquisition time in order to capture more B-scan images but would also result in more cumulated artefacts due to movement. If correction is made for the large transversal pixel size along the Y-axis, to achieve the normal pixel size of 15 □m in an aberrated eye, acquisition time would increase to over 4.5 s.
The disadvantage of this method is that the CM (or the en-face fundus image) is generated after (i) acquisition is complete and (ii) software evaluation, both steps requiring some time. As another disadvantage, as presented above, the transversal resolution along the sampling direction of B-scan repetition is coarser than the transversal resolution along the lateral direction in the FD-OCT image.
Other possibility is to produce an en-face cumulated image (microscopy or SLO) and then switch the system to acquire a fast B-scan OCT image. The operation can be sequential and not simultaneous because the reference beam has to be blocked when acquiring the CM (or SLO) image, otherwise the reference beam saturates the CM (SLO) channel or produces noise in this channel.
Another possibility is to divert light form the object towards a separate splitter, as disclosed in the U.S. Pat. No. 5,975,697 for a time domain OCT and for a CS-OCT system in US2008/0088852 A1 by J. Rogers and M. Hathaway. This method however reduces the amount of light used for generating the OCT images.
Therefore, the present invention seeks to overcome the above disadvantages, providing configurations and methods of operation, characterized by simultaneous parallel acquisition of the OCT information and of microscopy (eye fundus) information.